PS can't, nativly, resize depending on the format of the picture - e.g. "resize to 1000px on the longest side". Actions will do it but you need one for landscape and one for portrait.
I find this a bit annoying so after a bit of hunting I found this: -
http://www.digitalmsx.com/software.php
It's a JSX file which you can configure to resize to whatever you want on the largest side (or to a fixed size). It can also, optionally, add watermarks, text or a border to each image.
You can call the script from within an action which in turn lets you call the action from Bridge.
As for a step by step... Dunno if anyone else could be bothered to do one as it's quite a lot of work especially given the help with PS/Bridge covers the whole Actions and Batch Processing topics quite extensivly.